Lost circulation



LOST CIRCULATION Texas Company, New York, N. Y., a corporation of Delaware No Drawing. Application May 18, 1954, Serial No. 430,717

12 Claims. (Cl. 252-85) ,This invention relates to Well drilling. More particularly, this invention relates to a material and a method for correcting and/or preventing lost circulation of drilling fluid during well drilling operations.

in the drilling of Wells a drilling fluid is continuously circulated from the surface of the ground to the bottom of the hole, through the drilling bit, and back to the surface again for the purpose of lubricating the drill bit and drill string, cooling the drill bit, removing the cuttings from the bottom of the hole to the surface of the ground, and for providing sulficient hydrostatic pressure to hold loosely consolidated caving formations in place, and for imposing a hydrostatic pressure upon the formations penetrated by the well bore to prevent the escape of oil, gas or Water therefrom to the well bore during drilling operations. When drilling through shale or porous or fractured formations or other formations having a relativelyhigh porosity or permeability to drilling fluid a condition known as lost circulation or lost returns is sometimes encountered. When lost circulation occurs, the drilling fluid employed in the drilling operation flows into a relatively porous formation traversed by the well bore with the resultant loss of drilling fluid. In certain instances the loss of drilling fluid is so great that all but a minor amount of the drilling fluid pumped into the Well bore is lost into the porous formation. In some instances the loss of drilling fluid is so severe as to warrant cessation of drilling until this condition can be corrected and even in some instances, abandonment of the drilling operations.

Various materials and methods have been suggested tocorrect and/ or prevent lost circulation. Various types of fibrous sealing materials have been added to the drilling fluid in order to prevent and/or correct lost circulation. These materials, however, may form a loosely adherent barrier or seal (wall scab) along the wall of the well bore at the zone of lost circulation which, when in place prevents lost circulation, is easily dislodged by the running in and out of the drilling tools. Certain other materials tend to plug the lost circulation zone or formation tightly in depth. This result is definitely objectionable if lost circulation occurred within the producing zone itself.

It is an object of this invention to provide an improved material and method for controlling and/or preventing lost circulation.

It is another object of this invention to provide a material suitable for overcoming lost circulation, which material is readily available in relatively large quantities and at relatively low costs.

It is still another object of this invention to provide an improved material and method of employing the same for preventing and/or overcoming lost circulation.

These and other objects of this invention and how they are accomplished will become more apparent with reference to the accompanying disclosure.

In accordance with this invention it has now been discovered that a material comprising an admixture of fibrous wood, ground leather and ground, flaky, tree bark, such as Douglas fir bark, is particularly effective for preventing lost circulation. Preferably the admixture of fibrous wood, ground leather and flaky bark contain these components in approximately the proportions of 1:1:1 by weight.

In accordance with this invention lost circulation of drilling fluid is prevented and/or corrected by admixing with the drilling fluid the above-mentioned materials,

either in admixture or separately, viz., fibrous wood,-

ground leather and flaky bark, and pumping the resulting mixture through the well bore to contact the zone of lost circulation. ground leather, flaky bark penetrates into the porous or permeable lost circulation zone a sufficient distance to form an effective seal to the flow of drilling fluid into these zones without the build-up of a thick filter cake on the Walls of the well bore at the zone of lost circulation, thereby avoiding freezing of the drill string. In

practice it is envisaged that the mixture of fibrous Wood, ground leather and flaky bark forms a mat around the well bore at the zone of 10st circulation. Essentially this mat is a sheath of lower permeability formed by the interlocking of fibers making up the added admixture. The formed mat usually has a permeability in the order of 10-100 darcys. The pores of the resulting mat are so small that even a bentonite-water mud forms a mud cake thereon, thereby efiectively sealing and stopping mud loss through the mat. Mud cake formation is necessary for effective sealing which will stop mud loss. Generally, a seal may be defined as being a mud-cake on a mat. A seal usually has a permeability less than 10 microdarcys.

The fibrous wood product employed in the lost circulation correcting admixture of this invention will be any suitable cellulosic wood product, preferably a chemically untreated wood product. Fibrous, shredded wood obtained from such Woods as Douglas fir, western hemlock,

Screen mesh: Percent by weight 10 58.4 20 17 .2"

Through 11.6

Patented Apr. 23, 1957 The admixture of fibrous wood,

The fibrous wood product usually will have a chemical composition similar to that of ground wood and will consist of cellulose, lignin and pentosans, generally in the proportions 60:25:10 respectively together with a minor amount, usually up to about by' Weight of the total composition, of associated materials such as fats, resins, waxes, tannins and sugars.

The ground leather employed in the admixture in accordance with this invention may be any type of tanned leather. The leather is ground or fiberized so that the resulting tiny fibers are hair-like in form so that the fibers when pressed or rubbed together tend to mat or form a felt-like material. Suitable fibers of ground leather may be obtained by grinding leather between abrasive wheels or similar fiberizing equipment. A particularly suitable ground leather product is sold by Magnet Cove Barium Corporation, Houston, Texa under the trade name Leather-Floc. Leather-Floc provides a thin, pliable mat which clings closely to the outline of the formation.

Theflaky bark product employed in the admixture in accordance with the practice of this invention is preferably a shredded, flaky Douglas fir bark product. A particularly suitable somewhat powdery, flaky, spongy Douglas fir bark product having a wide range of particlesize and shape is sold by the Weyerhaeuser Timber Company, Longview, Washington, under the trade name Silvaflake.

The materials of this invention, either in admixture with each other or separately, may be added to the drilling fluid within the mud pit itself, or at the inlet to the mud pump, or at any other suitable place. When the materials are added to the drilling fluid in rather large quantities, agitation of the drilling fluid together with the materials added thereto is desirable and helpful in order to prevent lumping of the added material and to obtain a better dispersion of the added materials within the drilling fluid. A most eflective admixture for correcting lost circulation has been found to be a uniform admixture of equal parts by Weight Simpson Treewool, Leather-Floc and Silvaflake.

The added materials in accordance with my invention may be considered either as preventative or curative with respect to lost circulation. A preventative material may be defined as a material suitable for preventing lost circulation by being continuously circulated with the drilling fluid. A curative material may be defined a a material employed for overcoming lost circulation or regaining circulation which is added to the drilling fluid substantially only at the time lost circulation occurs or when it is anticipated lost circulation will occur. It is preferred, however, in the practice of this invention that the admixture of fibrous wood, ground leather and flaky bark be added to the drilling fluid only at the time lost circulation occurs or when it is anticipated lost circulation will occur.

The material in accordance with this invention are added to the drilling fluid in the amountsnecessary to effectively seal off the lost circulation zone. It is realized that the required amounts will vary depending upon the thickness, permeability and porosity of the formations traversed by the well bore and giving rise to lost circulation. Of course, the material are added to the drilling fluid in amounts sufficient to overcome lost circulation. Preferably, the materials are added to the drilling fluid in amounts such that the amount of added material present in the drilling fluid is in the range 2-50 pounds per barrel of drilling fluid, more preferably in the range 5-20lpounds per barrel. Satisfactory results have been obtained when equal parts by weight Simpson Treewool, Leather-Flee and Silvaflake have been added to an aqueous drilling fluid in an amount of 6 pounds per barrel of drilling fluid.

The drilling fluid employed in the practice ofv this invention may be any of the conventional drilling fluids,

4 e. g. water-base, oil-in-water, water-in-oil, and oil base fluids.

The effectiveness of the materials of this invention for regaining circulation or preventing lost circulation has been demonstrated by conventional and well-accepted laboratory tests. Tests of the materials of this invention have been carried out on a modified apparatus designed for use in evaluating materials for regaining circulation and described in API code R. P. 29, third edition, May 1950. In accordance with this test a 1% inch layer of copper plated BB shot (0.173 inch diameter spheres) formed by pouring 800 grams of the spheres into a cell so that a bridge across the 4-inch holes is effected during sealing. This gives a filter bed having a porosity of approximately 35 percent and a permeability of approximately 70 percent darcys. Untreated mud is then poured on the filter bed in order to fill all void spaces, to wet all the surfaces of the spheres and to just cover the bed of spheres. An 800 ml. sample of mud of 30:2 centipoises viscosity is prepared. A weighed sample of the sealing material to regain circulation or to prevent lost circulation to be evaluated is added to the mud and the resulting mixture stirred thoroughly. The treated mud is then added to the testing apparatus. A pressure of p. s. i. is applied to the mud being tested. When the pressure has reached its maximum value, the quick-opening valve of the test apparatus is fully opened. Usually the test is of 30 minutes unless no seal-off is obtained. Both the volume of mud that passes through porous medium bed before a seal-off is accomplished and the volume obtained by normal filtration are measured and recorded. The thickness and character of the sealing material rnat build-up upon the BB shot filter bed is also noted and recorded. The same tests were carried out employing inch pea gravel in place of the BB shot. It was observed in all tests that an admixture of Simpson Treewool, Leather-Floc, and Silvaflake in the proportions 1:1:1 by weight produced the best seals.

The lost circulation materials for this invention were tested on a Sievers Seal Breakdown Pressure Tester. The seal breakdown pressure tester consisted of a 1%- inch diameter cell and a pipe 25 inches long. A perforated plate held a porous bed above the base of the cell which is equipped with a valve at each end and a pressure gauge. The lost circulation additive was mixed 1000 cc. of untreated mud and the resulting mixture poured into the cell onto the bed. The bottom valve is then opened and a pressure of 100 p. s. i. g. applied to the mud mixture. The mud spurt collected was weighed. Spurt volume as listed in the accompanying table which sets forth the resulting experimental data, is the quantity of mud passing through the bed until dropwise filtration started. The time required to form a seal was also noted. Pressure up to 1500 p. s. i. g. was then applied to the formed seal for five minutes. The additional fluid collected at the end of the test was called high-pressure spurt. The pressure at which seals broke was noted for those seals which did not hold the full 1500 p. s. i. g. pressure test. After completion of each test the apparatus was disassembled and the seal characteristics such as mat thicknes, plugging-in-depth and other pertinent data were recorded.

Seals produced by an admixture in accordance with this invention withstood a 1500 p. s. i. seal-breakdown pressure and the admixture of this invention was the best as judged by both types of tests in comparison with certain other lost circulation materials and mixtures then available. The pores formed by the mat produced by the admixture of this invention were so small that a bentonite-water mud formed a mud cake upon them, thereby sealing and eifectively stopping mud loss through the mixture maton the pea gravel bed. The resulting experimental data is set forth in the accompanying table.

drilling fluid.

lMud: California St0ek; Rogers Lake and MeKitti-ick Light. 1:1, 30i2 op. at 600 R. P. M viscosity; 705:2 lbs/cu. it. density; 1.5% sand content by volume] Conc., Total Mud High- Time Additive material lbs./ Mud Sport, Press. to Filter Bed bbl. Lost, g. gms. Spurt, Seal, gins. secs.

Additive A, a proprietary fibrous 6 540 355 185 23 34% in.

wood product. plea grave Additive B, a proprietary fibrous 6 Entire volume passed through Do.

wood product. at 10( p. s. i. A 1:1:1 mixture of fibrous wood, 6 525 255 270 14 Do.

ground leather and flaky tree bark, e. g., Simpson Tree-wool: Leather-Flee: Silvaflake. Additive A 6 535 315 220 35 BB shot. Additive B 6 Entire volume passed through Do.

at 10( p. s. i. A 1:1:1 mixture of fibrous wood, 6 350 150 200 12 Do.

ground leather and flaky tree bark e. Simpson Tree-wool: Leatherloo: Silvaflake.

Many substitutions, modifications and omissions are possible in the practice of this invention without departing from the spirit and scope thereof.

I claim:

1. A composition of matter comprising an admixture of fibrous wood, ground leather and flaky tree bark in the weight ratio 1:1:1, respectively.

2. A composition of matter comprising a substantially uniform admixture of fibrous wood, ground leather and flaky Douglas fir bark in the proportions 1:1:1 by weight, respectively.

3. A drilling fluid suitable for combatting lost circulation experienced in a well-drilling operation comprising a liquid and an admixture of fibrous wood, ground leather and flaky Douglas fir bark in the proportions 111:1 by weight, respectively, dispersed therein.

' 4. A drilling fluid suitable for combatting lost circula- 1 tion experienced in a well-drilling operation comprising a liquid and an admixture of fibrous wood, ground leather and flaky tree bark admixed therewith, said fibrous wood,

ground leather and tree bark being present in the weight ratio 1:1:1.

' the rangeZ-SO pounds per barrel of drilling fluid.

6. A drilling fluid composition in accordance with claim 4 wherein said admixture of fibrous wood, ground leather and flaky tree bark is present in the amount of 6 pounds per barrel of drilling fluid.

7. A drilling fluid composition in accordance with claim 3 wherein said admixture of fibrous wood, ground leather and flaky Douglas fir bark is present in an amount in the range 2-50 pounds per barrel of drilling fluid.

8. A drilling fluid in accordance with claim 3 wherein said admixture of fibrous wood, ground leather and flaky bark is present in the amount of 6 pounds per barrel of 9. A process for correcting lost circulation of drilling fluid during drilling of a well bore comprising admixing an admixture of fibrous wood, ground leather and flaky tree bark in the proportions 1:1:1 by weight, respectively, with said drilling fluid and pumping the resulting mixture through said well bore to contact the lost circulation zone within said well bore.

10. A process for correcting lost circulation of drilling fluid during drilling of a well bore comprising admixing with said drilling fluid an admixture of fibrous wood, ground leather and flaky Douglas fir bark in the weight ratio 1:1:1 and pumping the resulting mixture through said well bore to contact the lost circulation zone within said well bore.

11. A process for correcting lost circulation of a drilling fluid during drilling of a well bore comprising admixing with said drilling fluid an admixture of fibrous wood, ground leather and flaky Douglas fir bark in the weight ratio 1:1:1, said admixture being added in an amount between 2 and 50 pounds per barrel of drilling fluid, and pumping the resulting mixture through said well bore to contact the lost circulation zone within said well bore.

12. A process in accordance with claim 11 wherein said admixture is added in the amount of about 6 pounds per barrel of drilling fluid.

References Cited in the file of this patent UNITED STATES PATENTS Camobell et al June 10, 1952 Simon et a1. Apr. 7, 1953 OTHER REFERENCES 

1. A COMPOSITION OF MATTER COMPRISING AN ADMIXTURE OF FIBROUS WOOD, GROUND LEATHER AND FLAKY TREE BARK IN THE WEIGHT RATIO 1:1:1, RESPECTIVELY. 